Polymeric materials are widely used as membranes for the separation of gases and liquids and as barriers in packaging. In general, the fundamental molecular level mechanisms of transport have only been inferred from bulk transport measurements, i.e., permeation and diffusivity studies.
Chain dynamics play a crucial yet poorly understood role in the diffusional transport of small molecules through polymers. If the frequencies, amplitudes, and average separations of polymer chains were precisely known, a detailed molecular model of diffusional transport could be developed.
Various theories have been proposed to model transport processes in glassy polymers, the most notable being the so-called "dual mode" theory which describes transport and sorption of gases by polymeric systems in terms of two types of sites or modes. These sites are described as Henry's law sites (dissolution sites) and Langmuir sites (hole filling sites).
According to simple dual mode theory, the two sites have different diffusion coefficients which are independent of pressure. Both sites are assumed to have different binding strengths for gas molecules. Therefore, gas molecules in the two sites would have different mobilities, it being assumed that Henry (dissolution) sites would have less restricted mobility than the Langmuir (hole filling) sites.
The dual mode theory adequately describes the pressure dependence of such parameters as the solubility constant and permeability of a gas in glassy polymers, both of which are known to decrease with pressure for most glassy polymeric systems. However, the dual mode theory describes transport as a macroscopic phenomenon, neglecting the molecular properties or dynamics of the polymeric membrane.
One approach to the determination of molecular dynamics of a polymer involves the use of "magic angle spinning" (MAS) .sup.13 C NMR spectroscopy. This probing of microscopic molecular dynamics, as distinguished from the study of the macroscopic phenomenon described, requires spinning a sample cell at 1000 to 5000 Hz which imposes certain constraints on the procedure. For example, the cells must be well balanced, dimensionally stable, and cannot be connected to any external condition-establishing source, such as a pressure source. Therefore, in order to change the pressure in the cell, the procedure must be discontinued and the cell removed from the spectrometer. Maximum pressures safely achieved with this technique are under 5 atmospheres, well outside the range of interest for membrane applications.